Electroinc subscriber&#39;s loop telegraph repeater



J. R. DAVEY Aug. 18, 1953 ELECTRONIC SUBSCRIBERS LOOP TELEGRAPH REPEATER Filed July 12, 1950 4mm j" Il MA IIIII-I" /NVE/vron J. R. DA VEV /A f. d.

A r Trap/VE v Patented Aug. 18, 1953 ELECTRONIC SUBSCRIBERS LOOP TELEGRAPH REPEATER James R. Davey, New York, N. Y., assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application July 12, 1950, Serial No. 173,442

Claims.

This application pertains to telegraph repeaters and more particularly to an improved electronic hub-type subscriber loop telegraph repeater.

Acknowledgement is made of patent application Serial No. 123,366 by W. T. Rea, filed October 25, 1949, now Patent No. 2,594,993, which discloses an electronic hub-type subscriber loop telegraph repeater. The present telegraph repeater represents an improvement over that repeater in that:

1) The signals from the subscriber are received on a voltage basis rather than a current basis. This eliminates the necessity of balancing the capacity of a cable loop in order to prevent kickoii during transmission of signals from the central office. Kick-off tends to occur only during transmission into an inductive loop and can be prevented by including a shunting capacitance in the loop pad circuit.

(2) From the plate of a pentode tube, square wave, rather than peaked wave, current signals are sent into the loop. This results in near zero bias transmission and in a reduction in noise induced in adjacent loop conductors.

(3) During reception of signals from the subscriber, the loop capacitance is discharged on a constant current, rather than a constant voltage basis. This causes the loop capacitance to be discharged during a spacing signal at least as quickly as it charges during a marking signal, and hence no inductive loop loading is required at the subscriber station or elsewhere. Although the loop current falls slowly during a spacing signal, the voltage at the head of the loop, which is used to control the received signal, decreases rapidly.

(4) Feedback from the loop circuit to the screen circuit of the driving pentode is' employed to insure that the transitions of the signals sent into the loop are symmetrical. Thus bias is avoided in the outgoing signals.

A feature of the invention, therefore, is an electronic telegraph hub subscriber loop circuit in which the signals are received from the subscriber on a voltage rather than on a current basis.

Another feature of the invention is a pentode employed to transmit square wave, rather than peaked wave, signals into the loop. This reduces the noise induced in adjacent loop conductors and the symmetry of the square wave signals results in zero bias transmission to the subscriber station.

Another feature of the invention is that when receiving signals from the loop, the loop capacitance is discharged on a constant current rather than a constant voltage basis, so that the loop capacitance is discharged during a spacing signal at approximately the same rate as it charges during a marking signal. This eliminates the need for loop loading since, even though the loop current falls slowly at a mark-to-space transition, the voltage at the head end of the loop, which controls the received signals, falls rapidly; thus the bias effect found in current operated repeaters caused by prolongation of the marking signal current at a mark-to-space transition eliminated.

Another feature of the vinvention is the emiployment of feedback from the loop to the screen circuit of the driving pentode to insure that the transitions of the signals sent into the loop are symmetrical and thus to avoid bias in the signals to that loop.

These and other features of the invention will vbecome apparent from the following description when read with reference to the associated drawing which taken together represent a preferred embodiment in which the invention is presently incorporated.

In the drawing:

Fig. 1 shows an electronic hub-type subscriber loop telegraph repeater;

Figs. 2 and 3 show modifications of a portion of Fig. 1.

It is to be understood, however, that the invention is not limited to incorporation in the present embodiment and that it may be incorporated in other embodiments which will be suggested to those skilled in the art by the following. The codes of a set of tubes and the values of voltages and of resistances employed in the preferred embodiment are shown on the drawing. It is to be understood, also, that the invention is not limited to use of these particular tubes nor to the voltage or resistance Values employed in the preferred embodiment.

Refer now to Fig. 1 of the drawing which shows the improved electronic hub-type subscriber loop telegraph repeater of the present invention. At the left of the drawing is shown the hub. A regenerative repeater may or may not be employed as required. If a regenerative repeater is not required, the hub electrically is a single point to which the receiving leg and the sending leg of each individual repeater in a particularhub concentration group are joined in parallel. There may be any number from two toten or more such repeaters joined to the electrical hub point. Since regenerative repeaters are frequently retube.

Only one electronic loop repeater with its single individual receiving leg and its lsingle individual sending leg is shown in the drawing.

The receiving leg is connected to the receiving hub RH and the sending leg to the sending hub SH. Y

In order to obtain the desired potential swings on the hub for the various signaling conditions, it has been found desirable to employ a ptentiometer at the hubpoint. This is lshown at the upper left and since it is associated with only the receiving hub when a regenerator is `employed, it hasv been termed the receiving hub potentiometer RHP. One vor more polarity dis-I criminating resistances are connected with the elements of the potentiometer which may have any one of a number of congurations to afford the desired voltage swings.

The circuit to be described is arranged for half duplex operationin the sense that transmission is possible from the receiving leg of any i one repeater in the concentration of the sending legs of all of the other repeaters simultanously. More than one repeater, however, may not transmit simultaneously through the single hub so that their signals will be unmutilated.

Signals on the send hub SH are connected to the grid of tube V4 by means of potentiometer string Rl, R2, R3. Tube Vfl thus conducts ,for the marking condition and is cut ofi for the spacing condition on signals transmitted from the hub toward the loop. During mark a loop current of 62.5 milliamperes flows through the The resistance of the loop isadjusted so that the potential at the plate of V is -50 volts. The screen voltage of V4 is adjusted so vthe plate current will be 62.5milliamperes under this condition of platevoltage. It is to be understood that the conditions prevailing "during the intervals while the circuits are idle, as well `as during the transmission of a marking Signal fromtlie hub toward the station in the'loop and from the station in the looptoward the hub Aare identical.V During Vthe transmission of a spacing Signal from the vhub toward the station 'ein the loop, the rloop Acurrent falls to zero and ,the voltage at the plate of V12 rises to +130 volts.

When the loop is opened at the subscriber station to Vsend aspacing signal, from the station in the loop, to the hub at'the central office, the

voltage at'the plate of Vd falls to nearly 130 volts since V4 will normally be in a conducting condition. It will bel observed that --130v volts 4Vis lconnected to the cathode of tube V4 and +7130 volts'is connected to its anode, through resistors R6, R and R4 thetotal magnitude of which is 2.89 mag'ohms. Thedirect-current resistance of tube V4 when'activated, as it is presently, is low relative'to 2.89 megohms, so that the potential of the anodev of Vrtube V4, when a spacing signal is transmitted from the station in the loop to the hub at the'central cnice, is substantially that of its cathode or 130 volts. Re-

sistors R4, R5 'and-R6 connected to' +130 volt -battery constitute a high impedance 'power'v Vconsequently V2b to conduct.

.of Vl by means of potentiometer R1, R8. Tube Vl, which has its cathode connected to voltsgconducts Aduring this space condition and draws acurrent of 20 milliamperes through resistor R9V from the/receiving hub potentiometer circuit RHP. The hub voltage is thus lowered from the marking-value of +60 volts to the spacing value of '-30 volts. Since the receive and send `hubs 'are connected together either directly or through a regenerative repeater in half duplex service, it is necessary to prevent the space which appears on the hub as a result of an open loop from reaching the grid of Vil and interrupting the spaceA Ycondition toward the hub. This is accomplished by means of tube vV2 and tube V3i). The grid of tube V2a is lfed from three sources: the plate of Vl which swings negative to transmit a-space tc the receive hub, the plate of VSa which swings v'positive for aspace from the loop, and the-plate of V'2b. rvI-he ef'- fective swing from V3a is made nearly twice that from Vl. When a Space issent from the loop the net eiect at rthe-grid of V2@` is a `positive swing which causes conduction. The resulting negative swingv'at the 'plate of ll'a, Iis coupled to thel grid of V219 'and produces a cutoff condition in that Yhalf of the tube. This causes a positiveswing at the plate of V'Zb which serves to hold the vgrid of V20/in a'positivls condition even after the completion of the spacing interval. The positive swing at the plate of V2b is also coupled to the 'gridofr Vb. YThe cathode of this triode is 'connected to the junction of resistors Rl and'RZ and'is 'arranged to hold that point at the normal `mark Vvoltage regardless of the voltage on `the sending hub. A `space from the subscriber is thus-prevented from coming back toY tubejV even'if it has been delayed in passage through a regenerative repeater.

When a space occurson the receive vhub from some other connectingrepeater, 4the plate of'V! swings negative without any opposingpositive swing from Vila. This causes V201. to cutoff and The resulting negative swing of theplate of Vbserves to hold V2a cut off ,afterthe 'completion of the spacing interval. The negative'swing of the'plate of V2b also keeps V317 cut off .even tor a .space condition at the junctionof R111 land R'a'ndthus signals Yfon the send hub mayY passithrough V to the loop. t

This circuit has the' usualbreak features ne'cessary for a loop repeater.- I the l'sub'scriberloop is closed at the time "of arrival of kan incoming break at the hub point, the `transmission path from the send 'hub to YVllfis inimediatelycleared and the loop openedff'the lo'op is openat the time the break'arrivesiatthe hub, the lpath of V4 is not cleared untiltlielcop'is closed. `O`nce cuts off andthe b'reak'vis"trarlsmitted to the hub.

At the Ysame time 4VVit isheld markingthuspreventing interruptionbffthebreak. *."I`V4 is cut Off at the tfle oflitath"ofmtll'bekfthe break is not passed to the hub until V4 becomes conducting.

Attention is called to the fact that in sending from the subscriber station, as the transmitting contacts are opened and closed, signals are received at the central station as a result of changes in voltage rather than changes in current. The potential at the anode of tube V4 is -50 volts for marking and approximately -130 volts for spacing. This voltage swing is applied through the V3a tube grid resistor string R4, R5 and R6 to cause it to conduct for marking and cut off for spacing.

In the usual practice, signals from the loop are received at the central station by means of a magnetic polar relay having a line winding and a balancing winding. With such an arrangement it is necessary to prevent false operation of the relay, or kick-olf as it is called, when signals are transmitted toward the subscriber station. Kick-01T on capacitative loops is caused by the discharge of the charged line capacity back through the line winding on a mark-to-space signal transition. In order to prevent it various methods have been followed, all of which involve the addition of condensers and their arrangement in a manner to produce a discharge which balances or counteracts that of the line capacity. When, as in the present arrangement, signals are transmitted from the subscriber station on a voltage rather than a current basis, the current responsive polar relay is not employed and there is no capacity kick-off problem to be dealt with.

There is a problem encountered in the present arrangement, however, when the subscriber loop is inductive rather than capacitative in character. Most loops are capacitative so the condition is met in a small percentage of cases. However, when the loop is inductive and transmission is from the central station to the subscriber station, on a transition from spacing to marking the voltage of the anode of tube V4 is required to swing from +130 volts when tube V4 is cut off to approximately -50 volts when tube V4 conducts. Under both of these conditions tube Va is supposed to remain conducting in order to prevent the transmission of a spacing signal element to the receiving hub and out over theV system. An inductive loop delays the build-up of the marking current and during the build-up period the potential at the anode of tube V4 may fall sufciently negative to cut off tube V3a momentarily and transmit a short spacing pulse to the hub. In eiect it is analogous to kickof of an electromagnetic receiving relay.

The above-described short false space effect may be prevented by connecting the condenser Cl between the anode of tube V4 and ground Which tends to neutralize the eifect of an inductive loop. The condenser Cl prevents the potential of the anode of tube V4 from going suinciently negative to cut off tube V3a. The condenser CI is required only when the loop is inductive.

As mentioned in the foregoing, employment of a pentode tube as the sending element into the loop results in the transmission of current Waves which are essentially square. This is due to the anode current versus plate potential characteristic of a pentode. Beyond the knee of the current versus plate potential characteristic, the current rises only slightly above the horizontal over a considerable voltage range. As a result of this, the tops of the signal Waves are relatively ilat rather than peaked at the mark-to-space vtransition which would result in marking signal bias and in greater noise induced in adjacent loop conductors..

In the case of a magnetic relay subscriber loop repeater in which the Vopposing ends of the loop through the operating Winding of the receiving relay may be terminated in batteries of opposite polarities for markng, when the loop is opened at the subscriber station, for the transmission of a spacing signal, theloop current in a capacitative loop falls relatively slowly as the capacitance discharges. When the loop is closed again at the subscriber station for the transmission of a marking signal, the current flows into the loop quickly to charge up the capacitance. The effect of the delay of a mark-to-space transition relative to that of a space-to-mark transition is to shorten the spacing interval. To counteract this, inductive loading is employed to equally delay the space-to-mark transition.

In the present electronic repeater no inductive loading is required in a capacitative loop as the capacitative line discharge is on a constant current instead of an exponential basis. When signals are transmitted from the loop toward the central station, on a mark-to-space transition the die-away of the voltage at the plate of tube V4 is relatively steep. There is no protracted delay as in the discharge of a capacitative line on an exponential basis. When the loop is again closed for the transmission of a marking signal the voltage at the anode of tube V4 rises quickly, but there is no need to delay the rise, by means of inductive loading, since the fall of voltage on the mark-to-space transition was correspondingly fast. Therefore, no inductive loading of a capacitative loop is Vrequired in the present invention to compensate for shortened spacing intervals characteristic of the usual magnetic relay receiving element in the usual repeater.

Although the current Versus -plate potential characteristic of a pentode tube is relatively flat beyond the knee of the curve over a wide range of potential, there is nevertheless a small rise from the horizontal in the current curve asl the voltage increases. From the foregoing description it is apparent that in transmitting from the central station to the loop, on a transition from spacing to marking, the potential of the anode of tube V4 is +130 volts at the instant the spacing signal terminates, and that it changes to -50 volts shortly thereafter, and remains at this level throughout the interval of transmission of the marking signal element. At thebeginning of the marking signal interval, While the potential of the plate of pentode V4 is at +130 volts, and until it falls shortly thereafter and remains at the 5U-volt level, there will be a small current peak, dependent on the pentode V4 current versus plate potential characteristic. This peak may be flattened, with a consequent reduction in signal distortion, by the arrangement of Fig. 2 which shows a modification of a portion of Fig. 1 in Which a transformer is added, having a primary winding in series with the anode of tube V4 and the line, and a secondary winding in the screen grid circuit. o

With such a transformer, on a space-to-mark transition, as the plate current in tube V4 rises it causes a decrease in screen voltage. This tends to limit or flatten the overshoot of current at the beginning of the marking interval. As soon as the current curve is flattened the transformer has no further effect until the end of the iep/tassi asithe inarkingcurrent the screen grid is increased momentarily by the 'secondary winding. This, however, isy Without either beneficial 4or' adverse'eiect except that it makesV it only slightly moredi'icult to cutfoffftub'e V4 at the'beginning of the spacing interval.

A similar compensating eiect' isV obtained by capacity coupling between the plateand'screen circuitsY of the pentode as shown Fig. 3'. The decreasel of plate potential on av space-'to-mark transition causes adecrease inv screen potential and prevents the current from overshooting the steady state current condition; Since the plate potential wave in arlong'loop is not a square 'Wave (as is the current Wave)v .this-'methodof Fig. 3 is somewhat inferior tothe. method of Fig. 2. Itis, however, to be preferred from the standpoint' of simplicityv and size of equipment.

What is claimed is:

1'. An electronic subscriber loop direct-current telegraph hub repeater andv control circuit comprising a loop transmission: conductor,v a iirst space discharge device which transmitst toward the loop, an output circuit for said device, a sec ond space discharge". device which transmits direct-current signals towardthe'hub, and an'in-put circuit for said-seconddevice;-said'conductor connected directly to a junction between-said input and'said output circuits.

2. An electronic subscr-iber loop direct-current telegraphv hub repeaterand control circuit'having ahub, a loop conductor,- a nrs-t space discharge y device which'transmits:direct-current'signals toward'saidfhubganinpnt circuitffor said device, a second space! discharge device which' transmits toward saidv lcop'conductor,` an output 'circuit for said second dev-ice andfa' directfjunction with-said loop conductor;intermediate'saidinput and said output circuits:V

3:- A-n electronic-1subscriber'loop7 direct-current telegraph;v hub 'repeater' and controll circuit having'a sendingi hub and'a'receiving hub; a' branch extending from said sending hub 'toftheinput of a sendingspace discharge device; an output` of said device connected directly tofthef loop'conductorY of a'y subscriber loop circuit', means/for transmitting signals' fromA said sending hub throughsaid device; means responsive thereto :for transmitting? signals directly through' said loop circuit, a receiving space discharge' device having an input connectedrto SaidlOOp'andfan output connected to said-receivinghub, for transmitting Ysource generatdrfdirecuy.e5/:opening sndfcic'siiig the loop land meansfrespon'sivefto the activation and; mac'tivationfl of said-V device dorf-impressing' a 'first and. -a secbndpotential signaly condition on Saldhllb y 5i` Aneiectrpmcfsibsnberiqopciret-cmrent telegraph-"nubirepeater?andlcoritrorciresn"conrprising' a hub anda subscriber loop and station circuit, a space discharge device in said repeater circuit, said device having an youtput circuit connected to saidl loop for transmitting into said loop, means, comprising ay connection from said hub to an input circuit of said device,- for impressing potential changes on said device, means responsive to saidpotential changes' for transmitting current signals directly into said loop and to said station circuit, said signals characterized by a departure from iiatnes'sv at steady state level near transition points or said signals and a feedback l dirtct-current-signals,towardsaidhub, and eleinents insaid receiving device.connectedito'said\ circuit comprising a branch having an electrical energy accumulating element interconnecting said' output circuit to a control electrode in said device to tend to latten said signals. Y t

6. In a telegraph system, a telegraph repeater station, a telegraph subscriber station, a telegraph loop circuit interconnecting said' stations, a space discharge device at said repeater station, an output of said device connected in series with said loop for transmitting signals to' said loop, a controlv electrode in said device, and' a feedback circuit interconnecting said output tosaid elec'- trode to modify the characteristic signals transmitted by' said device to said loop, said feedback circuit comprising anv electrical energy accumulator feedback element.

7. A system in accordance with claim 6 having a transformer as the feedback element, said transformer having a primary winding connected in series with said loop and a secondary winding connected to said electrode.

8. A' system in accordance With claim '6: having a condenser and a resistance'in series, interconnecting saidvloo'p and said electrode' asy the feedback elements;

transmission circuit inl series, so as to transmit substantially unbiased signals over said circuit.

l0. In a signaling-system, a capacitative transmission circuitY of relatively low impedance, a signal transmitter in saidcircuitj means inv said transmitter'ior transmitting current and no-current signals over said circuit, a4 high impedance power source, said source comprising a" source `of potential, a very" high'- lumped impedance,

relative' to the impedance of said transmission circuit, and the output circuit of al rst space Adischarge device connected in series, said transmission circuit terminated'insaid-sourceu second'space discharge device signal receiver for receiving said current and no-current signals, said 'second device having an input circuit connected to said high impedance source, said second de'- vice" responsivev to the; potentials across said source. l

JAMESR. DAVE-Y'.-

, inferences casein' the sie' or this patent UNITED STATES PATENTS 'Number Name` Date 1,852,050- Hamilton` Apr; 5, 1932 '2,444,782- Lordf i July 6, 1948 12,508,879# ZagorY M ay 23, 1950 f liav'ey'x Oct.l` 3i", 1950 

